Communication systems are known to support wireless and wire lined communications between wireless and/or wire lined communication devices. Such communication systems range from national and/or international cellular telephone systems to the Internet to point-to-point in-home wireless networks. Each type of communication system is constructed, and hence operates, in accordance with one or more communication standards. For instance, wireless communication systems may operate in accordance with one or more standards including, but not limited to, IEEE 802.11, Bluetooth, advanced mobile phone services (AMPS), digital AMPS, global system for mobile communications (GSM), code division multiple access (CDMA), local multi-point distribution systems (LMDS), multi-channel-multi-point distribution systems (MMDS), and/or variations thereof.
Depending on the type of wireless communication system, a wireless communication device, such as a cellular telephone, two-way radio, personal digital assistant (PDA), personal computer (PC), laptop computer, home entertainment equipment, et cetera communicates directly or indirectly with other wireless communication devices. For direct communications (also known as point-to-point communications), the participating wireless communication devices tune their receivers and transmitters to the same channel or channels (e.g., one of the plurality of radio frequency (RF) carriers of the wireless communication system) and communicate over that channel(s). For indirect wireless communications, each wireless communication device communicates directly with an associated base station (e.g., for cellular services) and/or an associated access point (e.g., for an in-home or in-building wireless network) via an assigned channel. To complete a communication connection between the wireless communication devices, the associated base stations and/or associated access points communicate with each other directly, via a system controller, via the public switch telephone network, via the Internet, and/or via some other wide area network.
For each wireless communication device to participate in wireless communications, it includes a built-in radio transceiver (i.e., receiver and transmitter) or is coupled to an associated radio transceiver (e.g., a station for in-home and/or in-building wireless communication networks, RF modem, etc.). As is known, the transmitter includes a data modulation stage, one or more intermediate frequency stages, and a power amplifier. The data modulation stage converts raw data into baseband signals in accordance with the particular wireless communication standard. The one or more intermediate frequency stages mix the baseband signals with one or more local oscillations to produce RF signals. The power amplifier amplifies the RF signals prior to transmission via an antenna.
As is also known, the receiver is coupled to the antenna and includes a low noise amplifier, one or more intermediate frequency stages, a filtering stage, and a data recovery stage. The low noise amplifier receives an inbound RF signal via the antenna and amplifies it. The one or more intermediate frequency stages mix the amplified RF signal with one or more local oscillations to convert the amplified RF signal into a baseband signal or an intermediate frequency (IF) signal. The filtering stage filters the baseband signal or the IF signal to attenuate unwanted out of band signals to produce a filtered signal. The data recovery stage recovers raw data from the filtered signal in accordance with the particular wireless communication standard.
In an ideal wireless communication system, the RF signal received by the receiver exactly matches the RF signal transmitted by the transmitter. In practice, however, the RF signal received by the receiver has been distorted from the RF signal transmitted by the transmitter due to the frequency response of the channel carrying the RF signal, i.e., the channel response. For instance, multi-path effects, distance, transmit power levels, et cetera all effect the channel response and consequently, adversely affect the received RF signal.
To accurately recapture the data embedded in RF signals, a receiver includes a channel equalizer or channel match filter, which attempts to undo the adverse effects of the channel. To train the channel matched filter or the channel equalizer, a channel estimation function may be performed on known data contained in the RF signal. The recaptured data is evaluated with respect to the actual known data to determine the gain and frequency response of the channel, i.e., the channel response. From this evaluation, the channel estimation function is determined and is subsequently used to provide the channel equalization.
Needless to say, channel estimation is a complex and vital function for accurate wireless communications. Typically, channel estimation circuitry includes a multi-tap filter and a coefficient estimation circuitry, wherein the coefficient estimation circuitry determines the coefficients that shape the multi-tap filter based on the channel response. The channel estimation circuitry processes up to each tap of the multi-tap filter separately and at the chip rate (i.e., the rate of the direct sequence spread spectrum signal), which requires the processor(s) of the coefficient estimation circuitry to operate at very high rates. Even though the channel estimation circuitry is operating at very high rates, the processing takes multiple iterations to obtain the coefficients, which is time consuming and requires complex hardware. In some applications, there is not sufficient time to completely train the channel estimation circuitry even though it is operating at very high rates. For example, in an IEEE 802.11b application, the channel estimation circuitry uses the preamble of a packet to obtain the channel estimation. The preamble may include a long or short SYNC data unit. When the preamble includes the short SYNC data unit, which is 56 microseconds in length, current channel estimation circuitry cannot obtain a reliable channel estimation for an entire multipath channel in such a short time, so compromises are made. For instance, only the three most prominent paths of the multi-path channel are used, which reduces the accuracy of channel equalization.
Therefore, a need exists for a faster, more accurate, and less complex channel estimation method and apparatus for use in wireless communication systems.